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Current of electricity (Part 1)

Textbook Chp 17. Current of electricity (Part 1). Topics. Current Electromotive Force Potential Difference Resistance. Current. Symbol for current: I Units for current: Ampere (A) Definition: Rate of flow of electric charge Formula: I = Q/t Q is charge (in Coulombs, C)

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Current of electricity (Part 1)

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  1. Textbook Chp 17 Current of electricity (Part 1)

  2. Topics • Current • Electromotive Force • Potential Difference • Resistance

  3. Current • Symbol for current: I • Units for current: Ampere (A) • Definition: Rate of flow of electric charge • Formula: I = Q/t • Q is charge (in Coulombs, C) • t is time (in seconds, s) • Current is a vector, it has both magnitude and direction

  4. Direction of Current • If positive charges are moving, the direction of current is the same direction as the positive charges • If negative charges (e.g. electrons) are moving, the direction of the current is the opposite direction as the negative charges • Note: 99% of the time, it is electrons which are moving

  5. Worked Example 1 • 2.0 C of positive charge moved from left to right in 1.0 s. • (a) what is the current? • (b) what is the direction of the current?

  6. Worked Example 2 • 5.0 C of electrons moved downwards in 4.0 s. • (a) what is the current? • (b) what is the direction of the current?

  7. Worked Example 3 • A current of 2.0 A flowed for 0.3 s. How much charge did that current carry?

  8. Worked Example 4 • Simple Circuit Diagram: direction of current direction of electrons

  9. Did You Know? • (not in syllabus) • How much current does it take to kill a person? • Ans: 0.1 A, 0.0001 A if through the heart • How much current in a lightning bolt? • Ans: 40 000 A (on average) • Did you know that majority of people survive a lightning strike?? (10-30% mortality rate)

  10. Half-Time: Electricity Men? • Mohan (India) • http://www.youtube.com/watch?v=PRf9MqqkMDM • BibaStruja (Serbia) http://www.youtube.com/watch?v=PpIXNZjAvpA

  11. Voltage • In Primary School, you used the word “voltage” in electricity. • DO NOT EVER USE THIS WORD FOR O LEVELS • Actually this is not a wrong term, but O levels prefer you to differentiate between e.m.f. and p.d.

  12. Electromotive Force (e.m.f) • The electromotive force (e.m.f.) is a measure of a source of electrical energy (usually a battery) • Symbol: ɛ (epsilon) • Units: volts (V) • Definition: the work done by the source in driving a unit charge around a complete circuit

  13. Electromotive Force (e.m.f) • (not in syllabus) • A battery with e.m.f. 1 volt will supply 1 joule of energy to 1 coulomb of charge around a complete circuit • In equation form: ɛ = W/Q

  14. E.M.F. in series • Recall from primary school • when batteries are arranged in series, the e.m.f. add up

  15. Potential Difference (p.d.) • Symbol: V • Unit: volts (V) • Definition: Work Done to drive a unit charge through the component

  16. E.M.F. vs P.D. • e.m.f. is a quantity describing sources of electrical energy (i.e. they supply electrical energy) • batteries, electrical generators • p.d. is a quantity describing sinks of electrical energy (i.e. they use up electrical energy) • resistors, bulbs, etc.

  17. E.M.F. vs P.D. • How do I use a voltmeter ? • When I attach a voltmeter across a resistor, what am I measuring? • When I attach a voltmeter across a battery, what am I measuring?

  18. Resistance • Symbol: R • Units: Ohm (Ω) • Definition: the ratio of the potential difference across the component to the current flowing through it • Equation: R = V/I

  19. Resistance • Simple Circuit Diagram: • Resistance = (Voltmeter Reading )/(Ammeter Reading) • R = V/I A V

  20. Resistors in Series • If there are two or more resistors in series, the total resistance is given by: • Rtotal= R1 + R2 + R3 + …..

  21. Worked Example 5 • What is the total resistance of this arrangement of resistors? • Rtotal= 1+2+3 = 6.00 Ω 1 Ω 2 Ω 3 Ω

  22. Resistors in Parallel • When there are two or more resistors in parallel, the total resistance is given by: • 1/Rtotal= 1/R1 + 1/R2 + 1/R3 + …..

  23. Useful Hint! • Most questions only ask for you to calculate two resistors in parallel • It may be useful to memorize this equation: • Rtotal = R1R2/(R1 + R2) • Note: this equation may only be used for 2 parallel resistors. If 3 or more resistors, use back original formula

  24. Worked Example 6 • What is the total resistance of this arrangement of resistors? • Method 1: 1/Rtotal = ½ + ¼ = ¾ • Rtotal = 4/3 = 1.33 Ω(3 sf) • Method 2: Rtotal = R1R2/(R1 + R2) = (2)(4)/(2+4) = 8/6 = 1.33 Ω 2 Ω 4 Ω

  25. Important Concept • When a resistor is added in series, the total resistance always increases • When a resistor is added in parallel, the total resistance always decreases

  26. Problem Solving Strategy • For more complex arrangement of resistors, • break it down into parts and determine subtotals of resistance before finally combining to find total resistance

  27. Worked Example 7 • What is the total resistance of this arrangement of resistors? • Step 1: find the subtotal of the parallel resistors first • Step 2: add this subtotal to the other resistor in series • Ans: 3.71 Ω(3sf) 3 Ω 2 Ω 4 Ω

  28. Worked Example 8 • What is the total resistance of this arrangement of resistors? • Ans: 2.77 Ω 1 Ω 4 Ω 2 Ω 2 Ω 3 Ω 3 Ω

  29. Summary • I = Q/t • Conventional Currventvs Electron Flow • Electromotive Force • Potential Difference • Resistance • R = V/I • Resistors in Series • Resistors in Parallel

  30. No Quiz! Quiz will only be done after Part 2 is completed

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